BACKGROUND Haemoglobin A1C (A1C), as a parameter of long-term glycaemic control, has been adopted to guide diabetic therapy all over the world. However, falsely high or low A1C could be troublesome in daily practice. METHODS A 75-year-old male diabetic patient affected by a reasonably inferred life-long history of microcytic anaemia was found to have abnormally low A1C values in the previous 5 months. Subsequent laboratory assessment with brilliant cresyl blue staining and haemoglobin electrophoresis detected haemoglobin H disease as the underlying cause of both the microcytic anaemia and the disturbed A1C measurement. CONCLUSIONS Enhanced erythrocyte destruction such as in haemoglobin H disease could explain a falsely decreased A1C level very well. Upon facing a questionable A1C value, physicians dealing with diabetes should consider the possibility of undiscovered underlying causes rather than too tightly glycaemic control. CONCLUSIONS Haemoglobin A1C values lower than the normal range most likely do not mean too good a control of blood sugar in diabetic patients.Careful investigation to find the underlying causes is mandatory to provide well-qualified medical care.Various haemoglobinopathies with chronic haemolysis should be considered as the background reason, especially in an endemic area for thalassaemia.